A code division multiple access technology is used in a Direct Spreading CDMA (DS-CDMA) system. Due to different propagation delays for different signals and the existence of scramble codes, the spreading code sets adopted by the respective signal are not completely orthogonal to each other. Such an interference caused by a non-zero cross-correlation coefficient is usually called as Multiple Access Interference (MAI).
Usually, a Matched Filter (MF, where a traditional Rake receiver just conforms to the MF principle) or a Multi-User Detector (MUD) is used in a CDMA system to recover data before being spread and scrambled. The traditional MF device is impossible to suppress multi-access interference effectively, while the MUD can eliminate the impact of MAI in a better manner.
In a TD-SCDMA system, Joint Detector (JD) is adopted by the MUD. As a linear MUD, it needs to complete a system matrix inversion operation. As a result, when a large Spread Factor (SF) or a long scrambler is used by the CDMA system or there exist too many interference users, the dimensions of the system matrix will increase and the computation load of the matrix inversion will become unacceptable.
The existing multi-user detection technology cannot eliminate interference signals effectively, so there exists a need to improve such a technology.